This invention relates to a low voltage bias circuit for a photo-diode.
It is well known that a p-n junction biased with a reverse voltage behaves like a current source, generating a current that is proportional to light incident upon the junction. One circuit configuration that is particularly well suited for supplying a reverse bias and at the same time producing either an output current or an output voltage which, without any substantial offset, is proportional to the photocurrent, consists of a bipolar transistor, the photodiode being connected from the transistor emitter to a ground buss, a voltage bias being provided to the transistor base with respect to the ground buss in a polarity to forward bias the base-emitter junction and a signal sensing means at the collector of the transistor. The collector current substantially equals the photo-current with no off-set.
This conventional bias circuit, however, has the disadvantage that for situations wherein the incident light may change rapidly, there is an accompanying change in the base-emitter voltage drop and also sometimes in the non-ideal base bias voltage, which leads to a change in the magnitude of the reverse bias voltage applied across the photo-diode. Since all p-n junctions have an inherent junction capacity, such photo-diode voltage changes lead to a delay in the photo-diode current in response to a step-function of incident light.
It is therefore an object of this invention to provide a low-voltage bias circuit for a photodiode to maximize the photo-current response time to rapid changes in light intensity.
It is a further object of this invention to provide a feed-back regulated voltage across the photo-diode.